The formation of colored photographic images by the coupling of oxidized aromatic primary amino developing agents with color-forming or coupling compounds is well known. In these processes the subtractive process of color formation is ordinarily used and the image dyes are intended to be cyan, magenta, and yellow, the colors that are complementary to the primary colors. Usually phenol or naphthol couplers are used to form the cyan dye image, pyrazolone or 2-(alpha-cyanoacetyl)-coumarone couplers are used to form the magenta dye image, and open-chain reactive methylene couplers having two carbonyl groups attached to the active methylene group (which is often substituted with a so-called "coupling off" group such as halogen, aryloxy, thioheterocyclic, and the like) are used to form the yellow dye image.
In these color developing processes the color forming coupler may be either in the developer solution or incorporated in the light-sensitive photographic emulsion layer so that during development it is available in the emulsion layer to react with the color developing agent that is oxidized by silver image development. Diffusible type couplers are used in color developer solutions. So-called Fischer type couplers and nondiffusing couplers which are soluble in high boiling solvents are incorporated in photographic emulsion layers. When the dye image formed is to be used in situ, couplers are selected which form nondiffusing dyes.
Conventional color-forming couplers are often four-equivalent, that is, they require the development of four molecules of silver halide in order to form one molecule of dye. Two equivalent couplers require the development of only two molecules of silver halide to bring about the formation of one molecule of dye. Two-equivalent couplers are very desirable for color photography, since only about one-half the usual amount of silver halide is needed and the light-sensitive coatings can thus be made thinner. Certain of the available two-equivalent couplers form dyes upon reaction with oxidized color developing agent, which dyes are subject to a gradual loss of color intensity, called "fading" when they are exposed to light and/or heat for a prolonged period of time. The fading of a colored photographic image is undesirable and much effort has been expended to date in efforts to minimize image fading.